Certain problems of general significance can be approached directly in the relatively simple CNS of the leech. Our principal aims are to study (1) integrative mechanisms that underlie behavior, (2) the mechanisms responsible for changes in synaptic transmission which develop slowly over weeks or months following lesions to the CNS, (3) regeneration of connections by individual nerve cells after their processes have been severed. The advantages of the leech are that the neurons are few in number, connected in a stereotyped manner and give rise to simple reflexes. Much is now known about the properties, functions and connections of individual sensory and motor neurons and the movements they produce. We have recently demonstrated pronounced changes in synaptic transmission between sensory and motor cells following lesions to the CNS. Moreover, a similar series of changes occurs in ganglia maintained in organ culture. In such preparations a combination of electrical, chemical and morphological techniques can be used to analyze the underlying mechanisms. Similarly, if regeneration occurs in culture, as in operated animals, one can hope to ascertain what factor influence the growth of neurons and enable them to form specific connections with appropriate targets.